The term "water" as used herein includes within its scope any source of water which is liable to be impure or to contain bacteria or other organisms that are susceptible to being treated with ozone. The water may for example include solids. Indeed, it may be sewage, or sewage sludge or other aqueous effluent.
Ozone is an oxidizing agent which finds use in the treatment of water, for example to produce a potable product. Since ozone is relatively unstable compared with other widely used industrial gases, such as oxygen and nitrogen, it is produced in commercial practice at the site of its use. The production method is the creation of an electrical discharge in a volume of gas comprising molecules of oxygen. The electrical discharge creates excited and unstable species of oxygen that react with unexcited oxygen molecules to form ozone. Typically, the source of gas comprising molecules of oxygen is air. There are, however, advantages in using, instead, a source of oxygen that has been separated from air, that is to say either pure oxygen or oxygen-enriched air. Firstly, the yield of ozone obtained increases with an increasing oxygen partial pressure. Secondly, a smaller ozonator is required for a given rate of production of ozone.
Although oxygen is soluble in water to a significant degree, ozone has a greater solubility. It is therefore typical that the effluent gas from the treatment of water with ozonized oxygen includes an enhanced mole ratio of oxygen to ozone. In order for oxygen to be preferred to air as the feed gas of choice for preparing ozone, it is desirable to put to good use the oxygen that is not converted to ozone. For example, either the residual gas formed after contacting the water with the ozonated oxygen can be passed to another treatment process for use as a treatment gas or it can be recycled to an apparatus or plant which produces the ozone. Since the water that is being treated with the ozone normally contains dissolved air, nitrogen is displaced from solution by the ozonated oxygen and thus the residual gas comprises a mixture of oxygen and nitrogen together with any undissolved ozone and it is desirable that a "recycle" ozonation plant should be designed on a basis that takes account of this dilution with nitrogen of the oxygen that is not converted to ozone.
Various different systems have been proposed for recycling the vent gas from an apparatus in which ozone is contacted with water. A general summary of such systems is provided in Chapter VI entitled "Ozone Generation and its Relationship to the Economical Application of Ozone in Waste Water Treatment" by Harvey M Rosen, edited by Evans and published by Ann Arbor Science, 1972. In a first system, air is compressed and oxygen is separated from the compressed air by pressure swing adsorption. The resulting oxygen gas is passed through an ozonator and the resulting ozonated oxygen is contacted with water to be treated. The undissolved gas from the contactor is then recycled to the compression stage. In an alternative scheme, a stream of oxygen is compressed and dried, and the resulting dried oxygen contacted with the waste water to be treated. Upstream of such contact, the waste water is subjected to vacuum degassing. Accordingly, the undissolved gas from the contactor is relatively free of nitrogen. The undissolved gas is recycled to the compressor. The latter process suffers from the disadvantage that it is necessary to degas the water to be treated. The former process suffers from the disadvantage that in the absence of any degassing of the waste water, the recycle gas from the contactor typically contains in the order of 40% by volume of components other than oxygen, and in particular contains nitrogen, argon, carbon dioxide and water vapor. The resultant size of the apparatus or plant for separating the mixture of air and recycle gas by pressure swing adsorption is therefore considerably greater than it would be were the recycle gas to contain a much lower proportion of nitrogen.
In a paper entitled "Ozone Generation Utilizing a Purity Oxygen Feed Gas", by A Warakomski, Proceedings of the Ninth World Ozone Congress, New York 1989 (Editor: L J Bollky; Publisher: International Ozone Association) there is an assessment of the operation of an apparatus including a first stage for separating oxygen from air by pressure swing adsorption and a second stage for ozonating the resulting oxygen. The assessment includes a discussion of the operation of the apparatus as part of a `classical recycle system`. The paper identifies the relationship between the energy demand of the ozonator and the ozone concentration in the product gas for a number of different feed gas compositions. It is disclosed that for any particular ozone concentration in the product gas, the ozone generation energy demand decreases with increasing oxygen concentration in the feed gas. In addition, the adverse effect of decreasing feed gas oxygen concentration is said to be relatively small at low ozone concentrations in the product gas. Hence the adverse effect is relatively large at high ozone concentrations. Further, it is disclosed that the feed gas in the `classical recycle system` contains about 55 to 65% by volume of oxygen. This implies that the `classical recycle system` involves recycling the effluent gas from the contactor directly to the ozonator inlet without first performing any separation of this gas to increase its oxygen concentration. Accordingly, such recycle processes are unfavorable in terms of the economics of ozone production.
GB-A-1 521 166 discloses a process for the treatment of water, which comprises contacting a partial stream of the water under pressure in a packed column with an oxygen-ozone mixture from an ozonator, mixing the partial stream with the water to be treated and recycling oxygen which is not dissolved in the column to the ozonator. The pressure of the contact column is adjusted so that the ozonator, the content of nitrogen in the recycled oxygen is in an amount which permits substantially optimum operation of the ozonator. For example, the feed gas to the ozonator may include between 2 and 11% by volume of nitrogen. It is disclosed that typically a pressure of 5 bar is needed to provide a percentage of nitrogen in the recycle gas as low as about 15% by volume. The requirement to operate the ozone generator at super atmospheric pressure is however disadvantageous since it greatly increases power consumption and since it creates an inherent risk of ozone leaking into the atmosphere.
The present invention provides a method and apparatus for the treatment of water with ozone which reduces the disadvantages associated with the known processes discussed hereinabove.
According to the present invention there is provided a method of treating with ozone a volume of water, containing dissolved air, comprising separating oxygen from air in a first region and ozonating the resulting oxygen in a second region; countercurrently contacting the ozonated oxygen with part of the water to be treated thereby to form an ozonated stream of water and an effluent gas comprising firstly undissolved oxygen and secondly gas displaced from solution in the water by the ozonated oxygen; mixing the ozonated stream of water with another part of the water to be treated; and recycling a first stream of the effluent gas to the second region and a second stream of the effluent gas to the first region.
The invention also provides apparatus for treating with ozone a volume of water containing dissolved air, comprising means for separating oxygen from air; an ozonator for ozonating the resulting oxygen; a contactor for countercurrently contacting the ozonated oxygen with part of the water to be treated whereby in use to form an ozonated stream of water and an effluent gas comprising firstly undissolved oxygen and secondly gas displaced from solution in the water by the ozonated oxygen; a device for mixing the ozonated stream of water with another part of the water to be treated; and means for recycling a first stream of the effluent gas to the ozonator and a second stream of the effluent gas to the said separation means.
Preferably, oxygen is separated from the air by pressure swing adsorption. The oxygen is preferably not pure. In general, the penalty attached to the production of pure oxygen in terms of yield of oxygen and specific power consumption, that is the power consumption per volume of oxygen produced, more than counteracts any benefit to be gained in the ozonator from the absence of nitrogen or argon impurities. Typically, the product oxygen from commercially available pressure swing adsorption plants has a purity of from 90 to 95% and such a product is suitable for use in the invention. It must be borne in mind, however, that recycling the second stream of effluent gas in effect increases the oxygen concentration of the air that is separated by pressure swing adsorption and thereby makes possible a reduction in the size and capital cost of apparatus or plant required to give oxygen of a given purity.
Preferably, the choice of adsorbent and operating parameters for the separation of the air is such as to enable the air to be dried during the separation of oxygen therefrom.
Countercurrent contact between the ozonated oxygen and water may, for example, be performed in a packed column. Typically, the volume of water to be treated is divided into two streams, the first being contacted with the ozonated oxygen and the second being mixed with the ozonated first stream. In this way, by treating only part of the flow of water with the ozonated oxygen, it becomes possible to minimize both the amount of nitrogen and other gases that are stripped from the water and the amount of oxygen that is dissolved so as to produce a vent gas with a relatively high proportion of oxygen (typically at least 75% by volume).
The contacting device preferably has a plurality of stages so as to maximize the dissolution of the ozone. Typically it has from 3 to 10 theoretical stages, but it can alternatively have fewer such stages, say from 2 to 3. Typically, the vent gas contains less than 0.5% by volume of ozone.
Typically, the rate of flow of water through the contacting device is from 10 to 70% (and preferably 25 to 30%) of the total flow of water to be treated.
Residual ozone in the vent gas is preferably decomposed upstream of division of the vent gas into the first and second streams. Such ozone decomposition may be accomplished by known means. Preferably, liquid water is disentrained from the vent gas upstream of the decomposition of the ozone.
There is no need to operate the countercurrent contactor with a super atmospheric gas head space. Indeed, it is preferred to run the countercurrent contactor with a head space pressure below atmospheric pressure. Such an arrangement minimizes ozone loss to the ambient air. The sub-atmospheric pressure may be maintained by a downstream blower used to recycle effluent gas from the countercurrent contactor.
The first stream of effluent gas is desirably dried upstream of its ozonation. Thorough drying is desirable since even small quantities of water vapor have a markedly adverse effect on the production of ozone. Preferably the first stream of effluent gas is dried by temperature swing adsorption. The effluent gas from the countercurrent contactor may be divided into the first and second effluent gas streams upstream or downstream of the drier.
Preferably the method according to the invention is controlled so as to maintain a substantially constant oxygen composition downstream of where the oxygen is mixed with the first stream of effluent gas and upstream of where ozonation takes place. Preferably, the rate at which the first effluent gas stream is recycled is varied as necessary so as to attain such a constant gas composition.